Modifying a coded bitstream

ABSTRACT

Various implementations are provided for, for example, determining watermarking data, performing the watermarking, and processing watermarked data. At least one implementation watermarks AVC bitstream that uses CAVLC entropy coding. The watermarking data may be determined in a pre-processing stage and stored for later use in watermark a bitstream. One method accesses variable-length encoded data including an encoding of information identifying an actual reference used to predictively encode an image. The encoding of the information has a particular value. The method determines an alternative value that is different from the particular value, such that the alternative value is an encoding of alternative information that identifies a different reference from the actual reference. The alternative value is for use in watermarking the variable-length encoded data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/US2008/007441 filed Jun. 13, 2008 whichwas published in accordance with PCT Article 21(2) on Dec. 18, 2008 inEnglish and which claims the benefit of U.S. provisional patentapplication No. 60/934,634 filed Jun. 14, 2007 and PCT ApplicationPCT/US2007/023172 filed Nov. 2, 2007.

BACKGROUND

1. Technical Field

At least one implementation described herein relates to coding.

2. Description of the Prior Art

Watermarking typically involves modifying a portion of data in such away that the watermark can be detected at a later time. Variousdifferent types of data can be watermarked, including coded data.However, coding schemes are advancing and existing mechanisms formodifying coded data may not provide desired watermarking capabilitiesfor such advancing coding schemes.

SUMMARY

According to a general aspect, variable-length encoded data is accessedthat includes an encoding of information identifying an actual referenceused to predictively encode an image, the encoding of the informationhaving a particular value. An alternative value is determined that isdifferent from the particular value, such that the alternative value isan encoding of alternative information that identifies a differentreference from the actual reference. The alternative value is for use inwatermarking the variable-length encoded data.

According to another general aspect, an entropy decoder is for decodingvariable-length encoded data including an encoding of informationidentifying an actual reference used to predictively encode an image.The encoding of the information has a particular value. A watermarkgenerator is coupled to the entropy decoder and is for determining analternative value that is different from the particular value, such thatthe alternative value is an encoding of alternative information thatidentifies a different reference from the actual reference. Thealternative value is for use in watermarking the variable-length encodeddata.

According to another general aspect, watermark data is for use inwatermarking variable-length encoded data that includes an encoding ofinformation identifying an actual reference used to predictively encodean image. The encoding of the information has a particular value. Thewatermark data includes an alternative value that is different from theparticular value, such that the alternative value is an encoding ofalternative information that identifies a different reference from theactual reference. The alternative value is a substitute for theparticular value for use in watermarking the variable-length encodeddata.

According to another general aspect, variable-length encoded data isaccessed that includes an encoding of information identifying an actualreference used to predictively encode an image. The encoding of theinformation has a particular value. An alternative value is accessedthat is different from the particular value, such that the alternativevalue is an encoding of alternative information that identifies adifferent reference from the actual reference. The alternative value isinserted into the variable-length encoded data as a substitute for theparticular value to watermark the image.

According to another general aspect, variable-length encoded data is foran encoding of an image. The variable-length encoded data includes anencoding of information identifying an alternative reference that isdifferent from the actual reference used to predictively encode theimage. The information identifying the alternative reference provides awatermark for the image.

According to another general aspect, variable-length encoded data for anencoding of an image is decoded. The variable-length encoded dataincludes an encoding of information identifying an alternative referencethat is different from an actual reference used in predictively encodingthe image. The information identifying the alternative referenceprovides a watermark for the image.

According to another general aspect, a pixel-domain image includes adetectable feature arising from a watermark of a variable-lengthencoding of one or more syntax elements from a predictive encoding ofthe image. The variable-length encoding included an encoding ofinformation identifying an alternative reference that is different froman actual reference used in predictively encoding the image. Theinformation identifying the alternative reference provided the watermarkfor the image.

According to another general aspect, a predictive encoding of awatermarked version of an image is variable-length encoded. Thevariable-length encoded data includes an indicator of a currentreference used in encoding the watermarked version of the image and anindicator of a resulting residue. The watermarked version of the imagehas a detectable characteristic that provides a watermark. Thedetectable characteristic results from (1) modifying a previousvariable-length encoding of a previous predictive-encoding of the image,the previous predictive-encoding being based on a previous reference andincluding information identifying the previous reference, wherein themodifying includes distorting the information in order to indicate analternative reference and to provide the detectable characteristic forthe image, and (2) decoding the previous variable-length encoding andthe previous predictive-encoding of the image to produce the watermarkedversion of the image in which the detectable characteristic is present.

According to another general aspect, information is accessed thatidentifies a location in variable-length encoded data at which avariable-length encoding of information is located. The informationidentifies one of at least two references associated with a predictiveencoding of an image. The encoding of the information is accessed at thelocation in the variable-length encoded data. It is determined, based onthe encoding of the information, which of the at least two references isidentified. Payload information is determined based on a result ofdetermining which of the at least two references is identified.

According to another general aspect, a pixel-domain image is accessedthat includes a detectable feature arising from a watermark of avariable-length encoding of one or more syntax elements from apredictive encoding of the image. The variable-length encoding includedan encoding of information identifying an alternative reference that isdifferent from an actual reference used in predictively encoding theimage. The information identifying the alternative reference providedthe watermark for the image. Detection metadata is accessed thatindicates a location in the pixel-domain image at which the detectablefeature is located. The pixel-domain image at the location is analyzedto determine a value for the detectable feature. Additional detectionmetadata is accessed that indicates a comparison value for thedetectable feature. The determined value for the detectable feature iscompared with the comparison value. Payload information is determinedbased on a result of the comparing.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Even if described inone particular manner, it should be clear that implementations may beconfigured or embodied in various manners. For example, animplementation may be performed as a method, or embodied as an apparatusconfigured to perform a set of operations, or embodied as an apparatusstoring instructions for performing a set of operations, or embodied ina signal. Other aspects and features will become apparent from thefollowing detailed description considered in conjunction with theaccompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an implementation for watermarking anentropy-coded stream.

FIG. 2 a is a block diagram showing an ordering of operations used inmodifying an encoded bitstream.

FIG. 2 b is a block diagram showing another ordering of operations usedin modifying an encoded bitstream.

FIG. 3 illustrates four prediction modes.

FIG. 4 is a table showing macroblock types for one particular standard,and showing additional information that is determined by the macroblocktypes.

FIG. 5 is a table showing an exp-Golomb coding provided in oneparticular standard.

FIG. 6 is a flow diagram of an implementation for determining detectionvalues for alternative intra-prediction modes.

FIG. 7 is a flow diagram of an implementation for determiningwatermarking metadata.

FIG. 8 is a flow diagram of an implementation for determiningalternative values for a reference indicator.

FIG. 9 is a flow diagram of an implementation for inserting replacementvalues.

FIG. 10 is a flow diagram of an implementation for inserting alternativevalues for a reference indicator.

FIG. 11 is a flow diagram of an implementation for identifying a payloadfrom watermarked data.

FIG. 12 is a flow diagram of an implementation for identifying a payloadfrom watermarked image data.

FIG. 13 is a block diagram of an implementation for watermarking contentand for detecting a payload.

DETAILED DESCRIPTION

There are multimedia watermarking applications in which a watermarkembedder must directly modify an entropy-encoded bitstream to obtain awatermarked bitstream that remains compliant with both the entropycoding scheme and the underlying data format (i.e. compressionstandard). It is often also required that the watermarking process mustnot introduce perceptible artifact in the multimedia data and that theembedded payload be recoverable even after decompression and processing.For many applications the watermark embedding process can have access tosome embedding metadata which is typically precomputed in an analysis orpre-processing stage. At least one implementation described hereinrelates to the embedding process, the generation of such embeddingmetadata for that embedding process, and the recovery of the embeddedpayload for that embedding process.

At least one implementation provides a watermark to an H.264/MPEG-4 AVCbitstream by modifying an entropy encoded bitstream directly withoutrequiring entropy decoding and entropy re-encoding. The modificationprovides the watermark. In one such implementation, the entropy code isa coding scheme known as Context-based Adaptive Variable Length Coding(CAVLC), which is widely used in the H.264/MPEG-4 AVC standard (“AVC”).

AVC is also referenced as the International Organization forStandardization/International Electrotechnical Commission (ISO/IEC)Moving Picture Experts Group-4 (MPEG-4) Part 10 Advanced Video Coding(AVC) standard/International Telecommunication Union, TelecommunicationSector (ITU-T) H.264 recommendation. CAVLC may also be used inextensions to the AVC standard. A first such extension is a scalablevideo coding (“SVC”) extension (Annex G) referred to as H.264/MPEG-4AVC, scalable video coding extension (the “SVC extension”). A secondsuch extension is a multi-view video coding (“MVC”) extension (Annex H)referred to as H.264/MPEG-4 AVC, MVC extension (the “MVC extension”).

In at least one implementation involving AVC, a macroblock is predictionencoded and the syntax elements of the prediction encoding aresubsequently entropy encoded using CAVLC. For intra-prediction, asopposed to inter-prediction, a macroblock is prediction encoded withoutrecourse to another picture. That is, without using a reference picture.Instead, the macroblock is prediction encoded with respect to itself orto another macroblock within the same picture (a picture may be, forexample, a frame or field). One of the syntax elements forintra-predicted macroblocks is the macroblock type, which indicates theintra-prediction mode as well as other information. In at least oneimplementation, the macroblock type (the “old” macroblock type) for amacroblock is changed to a new macroblock type such that a differentintra-prediction mode is indicated for a particular 16×16 macroblock.However, all other information indicated by the old macroblock type isleft unchanged. Further, the new macroblock type (the syntax element) isCAVLC encoded using the same number of bits used to encode the oldmacroblock type.

The above change in macroblock types can be done to embed a watermarkinto the data. Further, the determination of the replacement macroblocktype and corresponding replacement CAVLC bits can be done off-line, andthen at a later time(s) when the content (for example, the video data)is accessed, the replacement of the CAVLC bits can be done in anefficient real-time manner that does not require entropy-decoding. Otherimplementations, however, perform all or part of the replacement afterentropy decoding has been performed, either during the process ofdetermining the replacement information, or in a subsequent stage.

In many applications there is an increasing market need for watermarkingof a compressed video stream. Early approaches decompressed the stream,applied the watermarking in the pixel domain, and then recompressed thenow-modified pixel domain data. An early advance was to use informationfrom the original compressed stream to simplify the recompression. Thiswas improved with techniques that partially decompress the stream byapplying entropy decoding and parsing of the coded bitstream. Afterentropy decoding, the watermarking algorithms worked directly on syntaxelements such as coefficients and motion vectors. After the stream wasmodified to represent the watermark data, the entropy coding wasapplied. We describe various implementations in this disclosure, atleast some of which apply the watermarking on an entropy encodedbitstream directly without the entropy decode and re-encode steps.

Watermarking a CAVLC stream according to at least some of the describedimplementations involves changing a coded syntax element in the CAVLCstream. In general, changing a coded syntax element will cause the codedbit length to change. At least one implementation addresses the case inwhich the coded bit length is held constant, but other implementationsallow the coded bit length to vary.

At least part of this disclosure focuses on how to generate metadata fora streaming watermark embedder. At least one implementation changes theIntra-prediction mode, and is robust to various modifications of thebitstream. Other implementations may modify the CAVLC encoding ofresidue coefficients. One advantage of modifying the Intra-predictionmode is that by changing a relatively few number of bits (for example, 7bits in an implementation described below) in a CAVLC bitstream, a largeand detectable (although not perceptible) change can be provided for agiven block. Such a change can also be designed so that it is robust tochanges in the coded bitstream. For example, if the coded bitstream isdecoded and then re-encoded using different parameters (for example,block sizes, or modes for individual blocks), the bitstream willpotentially be completely different but the original changes can stillbe expected to be detected by analyzing the decoded pixel values.

A brief discussion of CAVLC and Watermarking is provided to assist inthe understanding of various implementations. These discussions areoften specific. However, these discussions are not intended to becomplete and, further, every detail in these discussions may notnecessarily apply to all implementations described in this application.Additionally, these discussions include much material that is known tothose of skill in the art. However, these discussions will invariablyinclude material, or the organization of material, that is novel, eventhough such instances might not be pointed out specifically. Thesediscussions are not intended to limit the breadth of the application.Rather, these discussions provide one or more specific contexts, of manypossible contexts, to assist the reader in understanding theimplementations.

We now provide a discussion of CAVLC. The final step in many videocompression methods is the lossless entropy coding of the compresseddata. In AVC, CAVLC is commonly used for entropy coding. CAVLC is a wellknown coding technique and is part of a class of coding techniquesreferred to as variable length codes. In general, variable length codes,in contrast for example to fixed length codes, assign codewords ofshorter length to syntax element values that have, or are expected tohave, a higher rate of occurrence. CAVLC generally provides a separateassignment, between codewords and syntax element values, for each typeof syntax element. Variable length codes are a class of codes that fallswithin the family of entropy codes. Arithmetic codes, including CABAC(Context-based Adaptive Binary Arithmetic Coding), is another class ofcodes that falls within the family of entropy codes.

We now present a short discussion of the general field of watermarkingand the more specific area within this field in which one or moredescribed implementations apply. Those of ordinary skill in the art ofwatermarking will be familiar with much of the information presentedhere.

The phrase “digital watermarking”, as used herein, typically refers tomethods that modify a work of art (typically an image, motion imagesequence, or audio clip) according to some payload data, such that thefollowing conditions are satisfied:

1. The modified version of the work is perceptually indistinguishable tothe original version of the work, and

2. The payload data can be recovered from the modified version of thework at a later time.

A subclass of digital watermarking methods, referred to as “robustdigital watermarking”, introduces a third condition:

3. The payload data can be recovered from a distorted version of themodified version of the work, where the distortion may have beenintroduced by common signal processing and signal handling to which themodified work may have subjected (for example, compression, noisereduction filtering, color enhancements) or the distortion may have beenintroduced intentionally by an adversary attempting to render thepayload data unrecoverable.

There are many applications of robust digital watermarking including,but not limited to, the following:

1. Owner Identification: the watermark payload identifies the owner of awork.

2. Copy Control: the watermark payload indicates a copyright associatedwith a work. Devices for viewing, duplicating, recording, printing,distributing, or any other action can recover the payload data andrestrict action to those allowed by the copyright.

3. Transaction Tracking: the watermark payload identifies the recipientto whom the copy of a work was legitimately distributed. This can beuseful when the legitimate recipients do not receive the rights tofurther distribute the work. If an unauthorized work is discovered, theoriginal content owner can recover the payload from the unauthorizedcopy and identify the recipient responsible for the unauthorized use.

Those of ordinary skill in the art will recognize that there are manyother applications of robust digital watermarking.

Watermarking can be performed in the “baseband” or on compressed works.Baseband imagery or motion image sequences are, for example,pixel-domain representations. Baseband audio works are, for example,audio samples. Some applications require watermarking of a compressedwork. In this case, the output is also a compressed work. The last stepof compression is typically entropy coding and the first step ofdecompression is typically entropy decoding. The entropyencoding/decoding process is typically lossless, but the compressionprocess typically also includes a lossy process. One approach forwatermarking a compressed work is first to apply the entropy decoding,then apply the decompression to obtain a baseband representation. Thebaseband representation is watermarked and the resulting watermarkedwork is compressed and entropy coded. This approach can be timeconsuming and can result in degradation of perceptual quality due to therecompression.

To improve the perceptual quality and to reduce the computationrequired, information from the original compressed work, such as motionvectors, mode decisions, quality factors, and other information, can besaved during decompression and used during recompression. This meansthat the recompression need not perform any motion estimation (thussaving computation and/or time) and use of the original quantizationfactors and mode decisions can result in improved perceptual quality.

A class of watermarking algorithms called “compressed domainwatermarking” has emerged. These methods can be described as performinga “partial decode” prior to watermarking rather than the “full decode”previously described. Here the compressed work is first entropy decodedto expose the syntax elements of the compressed work. These can includecoefficients (block DCT (discrete cosine transform) or waveletcoefficients for example), motion vectors, picture types, predictionmodes, and many other syntax elements. The watermarking algorithm thendirectly modifies some of those syntax elements. Finally, the modifiedsyntax elements are entropy coded to obtain the modified compressedwork.

At least one implementation described in this application falls into anew class of watermarking algorithms which could be called “entropycoded stream watermarking.” These methods directly modify an entropycoded stream as illustrated in FIG. 1.

Referring to FIG. 1, a system 100 performs watermarking. The system 100may also be considered as a process. The original compressed stream isdirectly modified by stream modification 102, according to the Payload,and the output is the resulting marked compressed stream 103. Thepayload can be, for example, the serial number of a content (forexample, DVD) player, the model of the player, or other informationdesired to be used for later identification. The payload can be uncoded,or can be coded as shown in data coding unit 120. The streammodification process is informed as to (1) the location of all changesto be made and (2) the actual changes to be made. The streammodification process is informed of this information by metadata whichcan be generated during a preprocessing stage 104.

The metadata 105 is generated by entropy decoding 106 (and possiblefurther decoding if multiple layers of coding are used) the originalcompressed stream, and generating a watermark 108. The metadataidentifies the locations within the stream where changes are to be made(that is, for watermark generation) and indicates how the stream shouldchange in response to different payload symbols. Implementations of FIG.1 are described for which the metadata is generated by an analysis ofthe decoded syntax elements of the compressed work. One challengingaspect of a watermarking method such as this is the generation of themetadata. The following explains how this can be done for one or moreimplementations.

The previous paragraph and FIG. 1 suggest that there may be apre-processing stage 104. It is useful to point out the situation inwhich there are three important times. The first time is where/when thecompressed bitstream is analyzed to generate some metadata. The secondtime is where/when some or all of the metadata generated at the firsttime is used, along with a specific sequence of symbols, known as thepayload, to modify the bitstream. The third time is where/when some (orall or none) of the metadata generated at the first time is used toanalyze a modified bitstream, a motion image sequence obtained bydecompressing the modified bitstream, or a distorted version of themotion image sequence obtained by decompressing the modified bitstream.The purpose of this analysis, in the third time, is to recover thepayload.

In order to understand the context, consider the first time occurringprior to distribution of a work, the second time occurring duringduplication where the payload used for each copy uniquely identifiesthat copy, and the third time occurring after an unauthorized copy ofthe work has been found at which time the payload is recovered to revealwhich of the distributed copies was the source of the unauthorized copy.This example is just an illustration, and is not intended to suggest anylimits to the application of these concepts. In addition, while theremay be application-specific requirements, there is no technicalrequirement that the first time (the preprocessing) and the second time(the embedding) be different. Indeed, all three “times” may be the same.

As additional examples, we refer back to FIG. 1. The pre-processingstage may be performed during the authoring of a movie for eventualdelivery, for example. In one scenario, a process is performed toidentify locations of coded elements for which acceptable replacementvalues have been determined. The results of that process (for example,the locations of such coded elements and the acceptable replacementvalues) are stored in metadata that is included with the coded movie.For this example, consider two cases. In one case, each entry of themetadata identifies a location in the bitstream and one acceptablereplacement value. In a second case, two acceptable replacement valuesare identified in each metadata entry.

The stream modification may be performed during playback of the movie,in which, for example, the serial number of the player (for example, asoftware player, a set top box player, or a DVD player) is used as thepayload. The payload is used, for example, to determine whether toreplace the identified coded elements. For example, if a singleacceptable alternative is provided in the metadata, then a “1” in thepayload sequence may indicate that the original value be replaced withthe alternative value, and a “0” may indicate that the original value beretained. As another example, all identified locations may be replaced,and the payload may indicate which of two provided replacement values(for each location in the coded sequence) is to be used. For example, a“0” may indicate use of the first replacement, and a “1” may indicateuse of the second replacement. Note that the words “alternative” and“replacement” are often used interchangeably in this application.Further, the alternative (or replacement) may refer to a variety ofitems, depending on context. The alternative may be, for example, analternative value for a syntax element or an alternative value for codedbits representing a value of a syntax element.

The recovery of the payload may be performed at a geographic locationthat is unrelated to the geographic locations at which the streammodification occurred. In other words, the stream modification may takeplace in Toledo, Ohio and the payload recovery may take place inBurbank, Calif. The key is to have access to a document containing databased on a modified stream. For example, a modified stream may be copiedelectronically and put on a recordable DVD, or a modified stream may bepresented and then re-recorded and encoded, and then put on a recordableDVD. If this recordable DVD is acquired, then this recordable DVD can beanalyzed to recover the payload. Recovery of the payload may be assistedwith detection metadata (shown in FIG. 1). Detection metadata mayindicate, for example, the locations in the data stream wherewatermarking information is embedded, as well as the acceptablereplacement values. Detection metadata may also, or alternatively,indicate, for example, a location within a picture (for example, frameor field) in which the watermark information is intended to produce adetectable change in the pixel-domain data. In this latter scenario, thedetection metadata may also indicate the expected change(s), asexplained further below. Detection metadata may be produced at the sametime that the watermarking (embedding) metadata is produced, or at adifferent stage.

A “watermark” may refer, for example, to the embedded data that replacesthe original data in a data stream. A “watermark” may also, oralternatively, refer to the effect that is produced on a decoded,displayed video picture in the baseband imagery as a result of embeddingreplacement data in the data stream. Thus, the “watermark” may refer toone or more of the embedded data or the produced effect on the resultingbaseband data (for example, an image or an audio clip). For example, theproduced effect may be a change in the luminance of a macroblock that isdetectable but not perceptible to a viewer.

We now provide a discussion of CAVLC-based data embedding in anAVC-coded bitstream. One way to watermark a compressed stream is tochange the values of one or more syntax elements. A general requirementis that the changed compressed stream is still a valid stream (remainscompliant with the specific compression standard). This requirement isto be met when the compression technique uses a variable-length codeentropy coding technique. This is the case for AVC with CAVLC. Thechanged (that is, modified) syntax element will be encoded using CAVLC,and these modified CAVLC bits will replace the CAVLC-encoded originalvalues in the bit stream. These bits may be the same length (same numberof bits) as the CAVLC encoding of the unmodified syntax element, or mayhave a different length. However, whether the length is the same or not,there is no effect on the accuracy of the other CAVLC bits because theCAVLC encoding of a syntax element does not depend on prior orsubsequent syntax element encodings.

In various described implementations, there are two general steps tomodifying a CAVLC encoded bitstream, but the steps can be performed ineither order. In discussing these steps, reference is made to “Step 1”and “Step 2”, but these step numbers are not intended to imply orrequire any particular or specific order of performance of the same.FIGS. 2 a and 2 b include block diagrams 200 and 210 that each showthese two general steps.

Given a CAVLC encoded AVC bitstream, Step 1 is to build a mapping (202)from the target element to the original coded block. In one practicalimplementation, this is done by decoding the bitstream and keeping trackof which bitstream bits produce which syntax elements. For example, aCAVLC decoder may be applied to the compressed bitstream to expose thesyntax elements.

Step 2 is to search for one or more acceptable alternative values forthe target element (204). One way to search for acceptable alternativevalues is to examine all possible alternative values and determinewhich, if any, are acceptable. Each possible alternative syntax value isCAVLC coded to yield the alternative bits. In general, all possiblealternative syntax values are “acceptable alternative values”. However,various implementations have one or more additional criteria.

One criterion is CAVLC encoding length. Because CAVLC is avariable-length code, some alternative syntax values will change thelength of the CAVLC encoding. This is acceptable in someimplementations, but other implementations impose a requirement that theCAVLC encoding length remain the same before and after watermarking.Other criteria relate, for example, to fidelity and detectability, asdiscussed further below.

Two different orderings of these two steps 202, 204 are depicted inFIGS. 2 a and 2 b, respectively. Note that in FIG. 2 b the mapping andassignment in Step 1 (202) is only required for target elements forwhich at least one alternative syntax value was found in Step 2 (204).

The AVC video compression standard, like most video compressionstandards, achieves compression by predicting the values in a block ofpixels from the values in one or more previously coded blocks of pixels.The difference between the prediction and the actual values, oftencalled the residue, is then transform coded and quantized. The block orblocks used for the prediction, the reference blocks, can be from thesame picture or from different pictures. Blocks whose reference blockscome from the same picture are called Intracoded blocks or simplyIntra-blocks. In this case, the prediction is often calledIntra-prediction.

For the luminance samples, an entire 16×16 macroblock can beIntra-predicted as a whole or can be divided into 8×8 sub-blocks or even4×4 sub-blocks. Each sub-block will then be Intra-predictedindependently. For 16×16 luminance Intra-prediction, four modes aredefined in the standard. For 8×8 and 4×4 sub-blocks, nine modes aredefined for each type of block. The encoder typically selects theprediction mode for each block that minimizes the difference between apredicted block and the actual pixel values.

To simplify the discussion, we only discuss 16×16 Intra-prediction. For16×16 Intra-prediction, the following four modes are defined asillustrated in FIG. 3:

1. Mode 0 (vertical) 310—Extrapolation from upper samples (H). In thismode, the neighboring pixels bordering the upper edge of a block areused as a prediction for all pixels in the corresponding columns of theblock. That is, for a given column in the 16×16 block, the borderingpixel in the upper vertically-adjacent block is used as the predictionfor the entire column.

2. Mode 1 (horizontal) 320—Extrapolation from left samples (V). In thismode, the neighboring pixels bordering the left edge of a block are usedas a prediction for all pixels in the corresponding rows of the block.That is, for a given row in the 16×16 block, the bordering pixel in theleft horizontally-adjacent block is used as the prediction for theentire row.

3. Mode 2 (DC) 330—Mean of upper and left-hand samples (H+V). In thismode, the neighboring pixels bordering the upper edge of a block, andthe neighboring pixels bordering the left edge of the block, areaveraged to determine the mean. The mean value is then used as aprediction for all of the pixels of the block. That is, the mean isdetermined of all of the bordering pixels in the lefthorizontally-adjacent block and in the upper vertically-adjacent block.The mean of these pixels is then used as the prediction for all of thepixels of the block. Each pixel in the block will then have the sameprediction.

4. Mode 3 (Plane) 340—A linear plane is fitted to the upper andleft-hand samples H and V. In this mode, a plane is formed. The plane isbased on all of the bordering pixels in the left horizontally-adjacentblock and all of the bordering pixels in the upper vertically-adjacentblock. The plane provides the prediction for each pixel in the block.

Note that the modes for 8×8 and 4×4 Intra-prediction can be modified ina manner that is analogous to the modification described herein of the16×16 Intra prediction modes. Further, all 3 block sizes use additionalsyntax that is amenable to modification in an analogous manner to themodification of the mode.

To decode a block, the predicted block is generated with the availablepixels of previously decoded neighboring blocks as specified by theIntra-prediction mode of the current block. Then, the decoded residuepixel values are added to the predicted block. The result is the finaldecoded pixel block.B=P+R  (3-1)where B is the final decoded block of pixel values, P is the predictedblock, and R is the block of decoded residues.

The basic idea of using Intra-prediction for watermarking in thisimplementation is to change the Intra-prediction mode from one toanother while not modifying the residue data. That is, the residue, R,is left unchanged although it may no longer be an accurate residue forthe new predicted block. The result of changing the prediction mode willbe a different predicted block (referred to as P_(W)). As a result, thefinal decoded pixel block (now referred to as B_(W)) will be differentfrom it should have been (which is B). That is, B_(W)=P_(W)+R, and B_(W)does not equal B.

The difference between the original block of decoded pixel values (B)and the watermarked block of pixel values (B_(W)), denoted ΔB, will bethe same as the difference between the original predicted block ofpixels (P) and the watermarked predicted block of pixel values (P_(W)),denoted ΔP.ΔB=ΔP where ΔB=B _(w) −B and ΔP=P _(w) −P,  (3-2)and the subscript ‘w’ indicates the watermarked version of each.

By changing the Intra-prediction mode of a macroblock, the pixels ofthat block will change by ΔB. For this change to be appropriate forwatermarking, we require in at least this implementation that thefollowing three requirements (R1-R3) be satisfied:

R1: the change be detectable from the reconstructed imagery,

R2: the change be imperceptible in the reconstructed imagery, and

R3: the detection be robust to some predefined set of signaldistortions.

To this end, we evaluate the suitability of each possible change andselect only those changes that meet these requirements.

The second requirement can be interpreted in many ways. We can requirethat the reconstructed imagery have high visual quality or that it beindistinguishable from the original, unmarked imagery, or that theperceptibility of the changes fall below some threshold which may beapplication dependent. There are many ways people judge the fidelity ofa watermark. For the purposes of this disclosure, we allow anyappropriate fidelity measure to be applied to judge whether or not, orto what extent, a proposed change meets the fidelity requirements of theapplication.

Assessing the fidelity requirement (R2) may be done in various ways. Fora given target element, we need to assess or predict the perceptualimpact of replacing its original value with each of the alternativevalues. Recall that this assessment or prediction has full access to allof the syntax elements of the compressed stream and can use these topredict the masking effect of the underlying imagery and the sensitivityof the human visual or auditory system (in the case of video, we areconcerned with the human visual system) to the modifications. Details ofsuch a prediction are well known to those versed in the art ofperceptual modeling. Perceptual models are widely cited in both thecompression literature and the watermarking literature.

In addition to the use of compressed domain computational predictions ofperceptibility, we may have the luxury of being able to fully decompressthe stream to a baseband motion image sequence. This may be the case,for example, if the analysis is being performed as a pre-process. Inthis case, the computational predictions can examine the actual pixeldata. This may lead to more accurate predictions of perceptibility.Additionally, we may be able to assess the perceptibility of replacingan original syntax element value with an alternative value bydecompressing the original stream to obtain a reference motion imagesequence and then replacing the syntax element value with thealternative and decompressing the resulting stream to obtain a secondmotion image sequence. Many well known techniques can be used forassessing the perceptibility of the difference between the two motionimage sequences.

Finally, we may have the luxury to resort to subjective assessment. Ahuman viewer can assess the perceptibility of the difference between thereference motion image sequence and the modified motion image sequence.Human subjects can assess the degree to which a modification satisfiesRequirement R2 for a given application.

These are but some of the methods that can be used to determine which ofthe possible syntax element changes satisfy Requirement R2.

We now consider the first Requirement R1 from a general standpoint. Useof the alternative syntax value for the target syntax element is assumedto induce some measurable change when the modified stream is laterdecompressed to a motion image sequence. There are generally two kindsof measurable changes that can be induced, “direct” and “indirect”.

With “direct changes” the syntax element corresponds directly to ameasurable artifact of the motion image sequence. For example,modification of a luminance block DC coefficient will directly result ina measurable change in mean luminance of the corresponding decompressedblock. With “indirect changes” on the other hand the artifact measuredin the image sequence is only indirectly related to the modification inthe stream. For example, modification of a motion vector will result inthe wrong block being used as a prediction and will therefore lead toincorrect pixel data in the corresponding decompressed block. It may bedifficult to determine which motion vector was used, but the use of adifferent motion vector can impact other measurable artifacts. Themotion vector can be used to yield a reconstructed block with higher orlower mean luminance.

The measurable change of requirement R1 may assist in identifying thelocations and in seeing what changes were made in order to recover thepayload. This is particularly useful when a pirated copy is obtained. By“seeing” the changes that were made, the source of the pirated copy canbe obtained.

In one implementation, it is determined which motion vector syntaxelements have at least two alternative syntax values, such that use ofone of the alternative syntax values will yield a reconstructed blockwith higher mean luminance and use of a different alternative syntaxvalue will yield a reconstructed block with lower mean luminance. Thecoded blocks of bits corresponding to the CAVLC coding of each of thesetwo alternative values, along with the bit location within the CAVLCstream, are sent, as metadata, to the embedder. Based on the value ofthe corresponding payload bit, the embedder will replace the originalblock of bits with the block of bits corresponding to one or the otheralternative block of bits.

For example, the embedder chooses or selects the block of bitsassociated with the syntax value that decreases the mean luminance inthe corresponding block if the payload bit is a ‘0’ and chooses theblock of bits associated with the syntax value that increases the meanluminance in the corresponding block if the payload bit is a ‘1’.Clearly, we need at least one target element satisfying all of therequirements for each payload bit that is to be embedded.

The metadata may also contain the frame number and block number in thedecompressed motion image sequence where the change in luminance will beinduced. For further robustness, the preprocessing stage may also storethe original mean luminance of that block in the metadata. Thisinformation may be used at the time of recovery (for example, decoding)to identify the modified block and compare its mean luminance with theoriginal value stored in the metadata. In the example given, a meanluminance that is higher than the original value will imply that thecorresponding payload bit is a ‘1’ and a mean luminance that is lowerthan the original value will imply that the corresponding payload bit isa ‘0’.

For this implementation, the first and third requirements (that thechange be robustly detectable) require establishing a good feature thatcan be reliably measured in the decoded imagery and can be modified bychanging the Intra-prediction mode of a macroblock. One such feature isthe mean luminance of the macroblock. Each Intra-prediction mode changewill result in a change, ΔB, in the decoded pixel values. Note that ΔBis, in general, a matrix of values (that is, ΔB is also a block). ThisΔB may have a positive or negative average value (averaged over theblock) and the magnitude of the average can vary from one mode toanother. The sign of the change (for example, the average value for ΔB)can be used to encode (embed) data. Further, the magnitude of the changecan be used as an indication of the expected robustness of the change,with a large magnitude indicating more robustness against furthermodifications to the coded bitstream. More sophisticated measures ofrobustness are available and can be used.

A second feature that may be used to satisfy R1 and R3 is the varianceof the reconstructed block of pixels. DC mode (mode 2) is different fromthe other three Intra-prediction modes in that all the 16×16 pixelvalues are predicted with a single value, which is the mean of thereference pixels. It is expected that an AVC encoder will use this modewhen a block is smooth in nature.

If we change the mode from mode 2 to one of the remaining three modes,the variance of the resultant block (B_(W)) can be expected to increasewith respect to the original block “B”. On the other hand, if an encoderchose modes 0, 1, or 3, it is expected that this block has higherfluctuation to begin with (compared to the situation in which mode 2 wasinitially chosen). By changing the intra-prediction mode from modes 0,1, or 3 to mode 2, the variance of this block B_(W) will generallydecrease with respect to the variance of “B”. Again, the ΔB associatedwith a change of Intra-prediction mode may result in an increase ordecrease in the variance of the reconstructed block B_(W), and themagnitude of the variance change can vary depending on the beginning andending modes. The sign of the variance change can be used to encode(embed) data. Further, the magnitude of the variance change can be usedas an indication of the expected robustness of the change, with a largemagnitude indicating more robustness against further modifications tothe coded bitstream. More sophisticated measures of robustness areavailable and can be used.

We now discuss an implementation for changing intra-prediction modethrough the macroblock type. In AVC, the 16×16 Intra-prediction mode ofa macro-block is specified in the mb-type field. The mb-type field alsospecifies other parameters about this block such as coded-block-pattern.

Referring to FIG. 4, a table 400 lists the macroblock types (“mb-types”)for 1 slices and provides a list of mb-type values 410 with theirmeanings. This table 400, taken directly from the standard, is used inthis implementation to find mb-type values 410 that change theIntra-prediction mode 420 without changing the coded-block-patterns 430and 440 or the prediction block size 445.

In order to preserve syntax other than Intra-prediction mode 420 whenchanging the Intra-prediction mode 420, the change of the mb-type 410 islimited to the values that differ only in Intra-prediction mode 420. Forexample, an original mb-type of 11 (see row 450), indicates that theIntra-prediction mode 420 is 2 (DC) and that the coded-block-pattern forchroma 430 and luma 440 are 2 and 0 respectively. This mb-type 410 canbe changed to 9, 10, or 12 to change the Intra-prediction mode 420without changing the coded-block-patterns 430 and 440 or the predictionblock size 445.

The mb-type is entropy coded in the bitstream. If CAVLC entropy codingis used, the mb-type is encoded with exp-Golomb code. Exp-Golomb code isa variable length coding scheme. Some special applications, such as thewatermarking of authored DVD disks, require that the replacement (e.g.VLC) data have exactly the same length as the original (e.g. VLC) data.In this case, only mb-types that result in same number of (e.g. VLC)bits can be used to replace the original mb-type.

Referring to FIG. 5, a table 500 is an Exp-Golomb coding table, andlists the bit string form (left-hand column) 510 and the correspondingvalue range of the Exp-Golomb code (right-hand column) 520. Table 500 istaken directly from the standard. From this table 500, we find that,using the example from above, an mb-type of 9 requires 7 bits. This canbe determined by first noting that the mb-type of 9 occurs in the range520 given in the fourth entry 530 of the right-hand column 520 which hasthe value “7-14”. Then we note that the corresponding (fourth) entry 530of the left-hand column 510 shows 7 bits, which are 0 0 0 1 x₂ x₁ x₀. X₂x₁ x₀ may be assigned, for example, in order, with 000 being assigned tothe value 7 and 111 being assigned to the value 14.

Accordingly, in this implementation in which the length of the CAVLCstring is to remain the same before and after watermarking, the mb-typecan only be replaced with another mb-type also requiring 7 bits. In thiscase, mb-types 9, 10, and 12 all fall in the same range (7-14) and allrequire 7 bits. Thus, mb-types 9, 10, and 12 are all possible candidatereplacement mb-types. In this way, we can combine the VLC bit lengths ofFIG. 5 with the macroblock types of FIG. 4 to determine which mb-typescan be used to replace the original mb-type. The following rules areenforced in the above methodology:

1. The alternative mb-type should differ only in Intra-prediction mode.

2. The size of the bit string corresponding to the alternative mb-typeshould be the same as that corresponding to the original mb-type (ifconstant bit length is required).

In a previous section, we suggested that a potential change should beevaluated with respect to its impact on the fidelity of thereconstructed imagery and the robustness of its detectability. In thissection, we describe a method for combining those two effects into asingle cost value and the use of that cost value in selecting whichchanges to apply.

Referring again to FIG. 1, the input of the preprocessing is the AVCencoded bitstream. The output is the watermarking metadata. One of thefirst steps in processing the stream is to identify each intra-codedblock, the Intra-prediction mode of which can potentially be changed.For each of these intra-coded blocks, we identify all alternativeIntra-prediction modes that satisfy the bit-length requirement (if theapplication requires constant bit length), and that satisfy the firstrule above that the corresponding alternative mb-types differ only inIntra-prediction mode. Assuming a particular detection measure (such as,for example, the luminance mean, or variance of a block as previouslydiscussed), an additional step is to gather the original detection valueand all the alternative detection values. A “detection value” is a value(a number) of the detection measure that results from the encoding. Forexample, the value of the luminance mean for a block. An alternativedetection value is the value that would result from using each of thealternative Intra-prediction modes.

Referring to FIG. 6, a process 600 illustrates an embodiment fordetermining detection values when mean luminance is used as a detectionmeasure. One or more detection values may be determined for a givenblock. For example, if there are multiple alternatives for a givenblock, a detection value may be determined for each alternative. Typicalimplementations optimize among the determined detection values. Suchoptimization may include, for example, selecting the best alternativefor each block, and then selecting the top ten from among these bestalternatives. This example results in picking the ten blocks with thebest detection values, where “best” is determined according to aselected measure, for example. Another implementation considers, for agiven block, pairs of alternatives, and selects the best pair for eachblock, as is explained further below.

The process 600 includes calculating the mean luminance of anintra-prediction coded block 610. This is the original detection value,and the mean luminance may be calculated from the sum of the predictionand the decoded residue as discussed earlier with respect to Equation3-1.

The process 600 then includes calculating the mean luminance of theintra-prediction coded block for each alternative intra-prediction mode620. Operation 620 uses the alternative mode for the prediction. Themean luminance may be calculated using, for example, the prediction withor without the existing residue data.

The process 600 stores the frame number (or, more generally, the picturenumber), the block location, and all of the mean luminances 630. Thesemean luminances include the mean luminance resulting from the originalintra-prediction mode (operation 610) and the mean luminance(s)resulting from the alternative intra-prediction mode(s) (operation 620).

The process 600 proceeds to the next intra-prediction coded block 640and repeats the operations 610-630. The process 600 is performed for allintra-prediction coded blocks in a given picture. Other implementations,of course, need not be as exhaustive. Note that an intra-predictioncoded block may be, for example, a block from an intra-coded picture(for example, an I frame), or a block from an inter-coded picture (forexample, a P frame) if that block is intra-coded.

Using the information gathered in the process 600, an implementation maymake a determination directly as to which locations and whichalternative replacement values should be used to provide watermarking.Various decision processes may be used. Below we discuss animplementation of one such process. The implementation discussed belowuses, for example, the information gathered in the process 600 to selectlocations and alternative values that meet, for example, therequirements R1, R2, and R3 listed earlier.

The key properties to a watermarking algorithm are typically therobustness (R3, above) and the fidelity (R2, above). Fidelity is alsoreferred to as the imperceptibility. Often, these two properties ofrobustness and fidelity contradict each other. We introduce ananalytical method to specify a balance between these two.

Let C_(F) represent the fidelity cost. C_(F) is defined such that themore visible the watermark, the larger the value. We would like to makechanges that have very low fidelity cost. There are many methods formeasuring C_(F), and a user may select the method/measure best suited toa particular application—the present implementation is independent ofthe selection.

Let C_(R) represent the robustness cost. C_(R) is defined such thatchanges resulting in weaker robustness have a higher robustness cost.Weaker robustness means, for example, that the changes can becomeundetectable more easily. For example, if a picture is decoded and thepixel data is filtered, some changes may become weakened and possiblybecome undetectable. Again, we seek to make changes that have a lowerrobustness cost.

For each alternative change, we calculate both the fidelity cost androbustness cost associated with making the change. We then combine thesetwo costs to obtain a total cost for making the change. That total costis calculated as follows:C=αC _(F) +βC _(R)where it is often helpful for restricting α+β=1. In this case we canwriteC=αC _(F)+(1−α)C _(R).The parameter α, restricted to the range of 0 to 1, is used to controlthe tradeoff between fidelity and robustness. A larger value of α placesmore emphasis on fidelity while a lower value of α places more emphasison robustness. For a given α, the cost C for each alternative predictionmode can be calculated. Those for which C exceeds a threshold can bediscarded.

To satisfy R2 above, which is the imperceptibility requirement, animplementation may additionally use, for example, a threshold level forC_(F). For example, in one implementation, the value of C_(F) for analternative prediction mode is compared to a threshold. If the value ofC_(F) is greater than the threshold, then the alternative predictionmode will not be accepted as a possible replacement for the originalprediction mode.

In one particular implementation that uses the above formulation ofcosts, 16×16 macro-blocks are used. In a 16×16 Intra-predictedmacro-block, there are three available alternative Intra-predictionmodes and the original mode. For this example, we assume that all of thedifferent Intra-prediction mode values are valid alternatives in that wecan change the mode to any one without changing the coded-block-patternsor the coded bit length (if constant bit length is required). A reviewof FIGS. 4-5 will reveal that this assumption is not always met, but theassumption is useful for clarity and simplicity of the currentdiscussion. In addition, we will assume that the detection measure isthe mean luminance of the block.

To embed a data bit, two prediction modes can be selected: a first modewill be used to represent a ‘1’ and a second mode will be used torepresent a ‘0’. We assume a prediction mode resulting in a lower meanluminance will be used to represent bit ‘0’ and a prediction moderesulting in a larger mean luminance will be used to represent bit ‘1’.Then, there are six possible pair combinations: {P₁=(mode0, mode1),P₂=(mode0, mode2), P₃=(mode0, mode3), P₄=(mode1, mode2), P₅=(mode1,mode3), P₆=(mode2, mode3)}. (Note that some of these combinations willbe unavailable if some of the prediction modes are not validalternatives.) In this case, we need to calculate the fidelity androbustness costs, C_(F) and C_(R), associated with selection of eachpair combination. Those costs can then be used to compare each to theapplication requirements. For each pair combination, the C_(F) and C_(R)are evaluated as follows.

Let B denote the original block of pixels. Let ΔB₀ and ΔB₁ denote thepixel differences between the watermarked block and the original blockif bit ‘0’ or bit ‘1’ is embedded, respectively. If the originalIntra-prediction mode is used to represent a ‘0’ or a ‘1’ bit, thecorresponding ΔB is simply all zeros. In such a case, the fidelity costdepends only on the other mode that is selected. Assuming that there areK available pairs, the fidelity cost of pair k can be obtained by:C _(Fpk) =F(B,ΔB ₀ ,ΔB ₁)  (4-1)where the function F(•) incorporates any fidelity measure as discussedabove (for example, the Watson model) and k is an index running from 1to K. Typically, this fidelity function will evaluate ΔB₀ and ΔB₁independently, obtaining a fidelity cost value for each, and will returnthe larger of the two values, representing the “worse” fidelity of thetwo or the sum of the two values. The fidelity measure may also be, inpart or whole, a subjective measure based on user evaluation.

Let L₀ and L₁ denote the luminance (a single value in thisimplementation) of the block if bit ‘0’ or bit ‘1’ is embedded,respectively. Let BlkSz denote the Intra-prediction block size. Therobustness cost can be measured as a function of L₀, L₁, and BlkSz. Fora given luminance (or luminance change), a larger blocksize may providefor increased robustness. The robustness cost of pair k can be obtainedby:C _(Rpk) =G(L ₀ ,L ₁ ,BlkSz)  (4-2)where the function G(•) is any robustness measure as discussed above(for example, the magnitude of the variance change or the mean luminancechange). Again, a typical robustness measure will evaluate therobustness of each change independently and return the larger of the twocosts or the sum of the two costs.

Finally, the final watermarking cost of this pair is:C _(k) =αC _(Fpk) +βC _(RPk)  (4-3)Once the cost associated with each pair has been calculated, the pairwith minimum cost can be selected for that block and the correspondingcost can be assigned to C, the cost of changing the block.C=min(C _(k)) where k=1 . . . K  (4-4)Once the watermarking cost of each block has been established, the blockselection process can be simply picking blocks having the smallestwatermarking costs based on the payload requirement. For example, ifthere are 10 payload bits, then the 10 blocks with the smallestwatermarking cost can be selected. The 10 payload bits of this examplemay be generated, for example, by encoding one or more payload symbolsto increase robustness. Another implementation does not encode payloadsymbols/information to increase robustness, but merely represents thepayload information in binary form as payload bits.

Referring again to the implementation of FIG. 1, the watermarkingembedding metadata 105 contains the bit offsets identifying the locationwhere, in the bitstream, each modification will take place and thevalues to be used in the case that the payload bit is a ‘0’ or a ‘1’.Thus, when keeping track of an alternative change, we track the bitlocation in the input AVC stream where the VLC code describing thecurrent mb-type can be found. We also keep track of the specific bitstring (from the table of FIG. 5) that is used to represent eachalternative change. Thus, after selection (perhaps based on minimizingthe total cost), the system can write out the embedding metadata 105.For each entry in this file, there is a bit offset and two bit strings(for example, the VLC bit strings), one for each of the two possiblepayload bits, ‘0’ and ‘1’.

In addition, the preprocessing stage writes detection metadata (alsoshown in FIG. 1). This includes the frame number (or, more generally,picture number) and block location of each change as well as theexpected luminances (or other detection measure) of the watermarkedblock if it represents bit ‘0’ and ‘1’. Also note that implementationsmay include the embedding metadata in the detection metadata to allowfor detecting the presence of watermarking information in a stream priorto decoding.

As just suggested, detection may be performed at the bitstream level, inlieu of or in addition to performing detection at the pixel level. Byanalyzing a bitstream, it can be determined what payload bits areembedded at the designated bit offsets. However, as discussed earlier,if the bitstream has been processed by, for example, decoding and thenre-encoding, the resulting (encoded) bitstream may be completelydifferent from the original (encoded) bitstream. For example, there-encoding may use (as indicated earlier) different block sizes ormodes. In such a case, detection of the watermark from the bitstream maybe impossible, but detection of the watermark from the pixels (afterdecoding the new bitstream) is possible if the watermark was robustenough.

Referring to FIG. 7, a process 700 provides an implementation thatdetermines the watermarking metadata (embedding and detection) for 16×16intra-prediction coded macroblocks using a cost approach. The process700 includes the basic features of the process 600. That is, the process700 of determining watermarking metadata for macroblocks includesdetermining detection values for those macroblocks.

The process 700 may be performed, for example, for a given set ofmacroblocks in a picture, or for all macroblocks in a picture, or forall macroblocks in a sequence of pictures. In the following discussionof the process 700, an implementation is assumed in which the process700 is performed for all macroblocks in a picture.

The process 700 includes looping (710) over all 16×16 macroblocks in thepicture, and performing a set of operations (720-750) for each 16×16macroblock that uses a 16×16 intra-prediction mode (see, for example,FIG. 4). The set of operations includes operations 720-750.

For each macroblock using a 16×16 intra-prediction mode, the possiblealternative intra-prediction modes, if any, are determined (720). In oneimplementation, the alternatives must satisfy the requirements that thecoded-block patterns (430 and 440) and the prediction block size (445)remain the same, and the coded bit length of the macroblock type (410)remains the same (see FIG. 5).

For each possible alternative intra-prediction mode or group of modes,the total cost is determined (730). In one implementation, determiningthe total cost involves calculating the fidelity cost and the robustnesscost using the sequence of equations described above. A group of modesmay be, for example, a pair of modes as discussed earlier, or some othercombination of modes.

The best possible alternative intra-prediction mode, or group of modes,is determined (740). In one implementation, the best is determined usinga minimization function such as in Equation 4-4 above, which selects thepair with the minimum total cost. Other implementations use differentfunctions to determine the best possible alternative, and also mayidentify multiple “best” possible alternatives. For example, in animplementation discussed above, the “best” possible alternative is apair. The pair may include one alternative, or two alternatives. Intypical implementations of the process 700, if there are no possiblealternative intra-prediction modes, then there is no “best” mode.

The embedding metadata and the detection metadata are stored for thebest possible alternative intra-prediction mode or group of modes (750).In one implementation, the embedding metadata includes the bit offsetinto the stream and the coded alternative macroblock type(s), in whichthe alternative macroblock type(s) corresponds to the best possiblealternative intra-prediction mode(s). In one implementation, thedetection metadata includes the picture number (or frame number), themacroblock location, and the original and alternative detection valuesor some other reference detection value. In such an implementation, thedetection values may be computed during the operation 730 of calculatingthe total cost. For example, the detection values may be determined inthe process of calculating a robustness cost.

Various implementations reduce, or more generally filter, the metadataprior to, or subsequent to, storing the metadata. In one suchimplementation, the best alternative mode (or group of modes) for eachblock are rank ordered and only the top one-hundred are kept and storedbecause there are only one-hundred payload bits. A second implementationspecifies a minimum number of unchanged bits that must be between anytwo changes in the bitstream. This criterion in the secondimplementation may, for example, preclude using the best alternativemodes for two adjacent blocks. A third implementation specifies the size(in number of bits) of each particular change in the entropy-encodedbitstream. This criterion in the third implementation may, for example,dictate the size of the entries in the embedding metadata. Eachparticular change need not, of course, actually change the value ofevery bit that is replaced. In yet another implementation, for a givenblock, the metadata is only stored for the best alternative mode (orgroup of modes) if this best alternative also satisfies the requirementsR1, R2, and R3. Further, implementations may combine these and othercriteria.

In one implementation, embedding metadata includes the locations in thecompressed bitstream where the original coded block of bits can be foundand two blocks of bits, one to be used as a replacement block if thecorresponding payload bit is a ‘0’ and the other to be used as areplacement block if the corresponding payload bit is a ‘1’. In oneimplementation, one of these blocks is itself the original block of bitsand the other is an alternative block of bits. In this case, the list ofchangeable syntax elements can include those for which there is only oneacceptable alternative value. In another implementation, the two blocksare both alternative blocks of bits corresponding to different syntaxvalues. In this case, the list of changeable syntax elements can onlyinclude those for which there are at least two acceptable alternatives.

In one implementation, detection metadata includes the embeddingmetadata. With such information, the payload can be recovered directlyfrom a watermarked coded bitstream.

In one implementation, detection metadata additionally or alternativelyincludes the location in the decompressed motion image sequence wherethe bitstream modification will be detectable. This may be specified asa frame or picture number and a block number, macroblock number, pixelposition, or any other locator required for recovery. The detectionmetadata also may include an indication of the detection valueassociated with a payload bit of “0” and a payload bit of “1” or someother reference detection value.

Detection metadata can also include the value of the detection measurethat would have been measured if no change had been made. In otherwords, this can include the original value of the detection measure. Ina previous example, the detection measure is the mean luminance of aparticular block and the detection data can include the original meanluminance of that block. By including the original value, the detectionprocess can ascertain whether the detection value has increased ordecreased as a result of the watermarking. In one implementation,determining the metadata is performed during pre-processing.

In order to increase the robustness of the watermarking technique toglobal or local changes in brightness or contrast, detection data mayalso be saved for blocks that are not expected to be modified by thewatermarking process. For example, the detection data may include theoriginal mean luminance of blocks not changed by the watermark. Thedetector can then use these as a reference to determine if the entireimage, or at least the part of the image in the region of the reference,has experienced a change in luminance. If the measured luminance in thereference blocks does not match that recorded in the detection data, acompensation can be made prior to recovery of the payload. For example,the compensation to a mean luminance measure could be added to allstored detection values

Referring to FIG. 8, a process 800 provides a general implementationthat determines embedding metadata based on a reference. The process 800thus has some similarity in functionality with the process 700.

The process 800 includes accessing variable-length encoded data thatincludes a reference indicator (810). That is, the variable-lengthencoded data includes an encoding (the reference indicator) ofinformation identifying an actual reference used to predictively encodean image. The reference indicator, which is the encoding of theinformation identifying the actual reference, has a particular value. Inone or more previously described examples, the reference indicator isthe encoding of a value of the macroblock type field that identifies,through the corresponding intra-prediction mode, the reference used topredictively encode a macroblock.

The process 800 includes determining a location in the variable-lengthencoded data of the reference indicator (820). The location is used, forexample, in creating the embedding metadata.

The process 800 also includes determining an alternative value for thereference indicator (830). The alternative value is different from theparticular value. The alternative value is an encoding of alternativeinformation that identifies a different reference different from theactual reference. All or part of the process 700 may be used, forexample, in performing the operation 830. In one or more previouslydescribed examples, the alternative value is the encoding of a differentvalue for the macroblock type field.

The process 800 may include various additional operations, such as, forexample, designating the alternative value as a substitute (a watermark)for the particular value, evaluating the effectiveness of thealternative value as a watermark, or creating embedding metadata.

After creating a set of possible watermark replacements using, forexample, the process 700 or the process 800, an implementation mayselect a subset of macroblocks to change. This selection is typicallybased on the payload requirements of the application, and any datacoding applied to the payload. The selection is typically informed bythe costs of each change (for example the fidelity and robustness costsas previously described). Usually, the selection will favor lower costchanges. In one implementation, the macroblocks are ranked according tothe total cost for the best possible alternative for each macroblock(730 and 740). The macroblocks with the lowest total cost (for the bestpossible alternative) are selected for watermarking with the payloaddata.

Referring to FIG. 9, a process 900 is shown that describes animplementation for inserting replacement values, such as, for example,inserting a watermark. The process 900 includes accessing a payload(910). The accessed payload may be in bits or other symbols. Typically,the payload is in bits or is converted into bits. Additionally, thepayload may be encoded.

The process 900 includes determining the replacement locations andreplacement values (920). Operation 920 may be performed using, forexample, the process 700, followed by a ranking process as justdescribed. Operations 910 and 920 may be performed in either order.However, one implementation performs operation 910 first and alsodetermines the number of payload bits, and then determines only theneeded locations in operation 920 by, for example, selecting only thetop ranking macroblocks until all payload bits have been assigned amacroblock. The operation 920 may be influenced by various otherimplementation requirements such as, for example, those discussed withrespect to FIG. 7.

In at least one implementation, all or part of operation 920 isperformed during a pre-processing stage. For example, in apre-processing stage, a table may be prepared that includes embeddingmetadata. Then, in operation 920, the replacement values and locationsmay be determined by accessing the table.

The process 900 further includes inserting the replacement values, atthe replacement locations (930) to provide, for example, a watermark.Inserting the replacement values involves replacing the existing codeddata with the replacement values. The modified data may then be storedor transmitted, for example. These replacement actions are typicallyinformed by a coded payload.

Referring to FIG. 10, a process 1000 is shown that describes a generalimplementation for inserting substitute values based on a reference. Theprocess 1000 thus has some similarity in functionality with the process900.

The process 1000 includes accessing variable-length encoded data thatincludes a reference indicator (1010). That is, the variable-lengthencoded data includes an encoding (the reference indicator) ofinformation identifying an actual reference used to predictively encodean image. The reference indicator, which is the encoding of theinformation identifying the actual reference, has a particular value. Inone or more previously described examples, the reference indicator isthe encoding of a value of the macroblock type field that identifies,through the corresponding intra-prediction mode, the reference used topredictively encode a macroblock.

The process 1000 includes accessing an alternative value for thereference indicator (1020). The alternative value is different from theparticular value. The alternative value is an encoding of alternativeinformation that identifies a different reference different from theactual reference. The alternative value may be, for example, accessedfrom storage or computed in real-time. An implementation may, forexample, access the alternative value from a table that is indexed by alocation of the reference indicator.

The process 1000 includes inserting the alternative value as asubstitute for the reference indicator (1030). The alternative value isinserted into the variable-length encoded data to watermark the image.

The process 1000 may include various additional operations, such as, forexample, storing or transmitting the modified variable-length encodeddata.

All or part of the data identifying locations of coded elements andpossible replacement values, as accessed by 1020, can be stored on astorage device, or sent electronically. One implementation stores thelocations and values on an apparatus such as a DVD, a hard disk, orother storage device. The apparatus includes a processor-readable mediumhaving information stored thereon. The stored information identifies areplacement value for a portion of an encoded set of data. Anotherimplementation provides a signal formatted to include the informationthat is stored on a processor-readable medium of this apparatus.

Additional implementations are directed to the result of variousprocesses, such as, for example, results of the process 900 or theprocess 1000. For example, one implementation is an apparatus such as aDVD, a hard disk, or other storage device that includes modified encodeddata, and another implementation is a signal formatted to include suchmodified encoded data. More specifically, one implementation is anapparatus that includes a processor-readable medium, theprocessor-readable medium having stored thereon encoded data including amodified portion. Another implementation is a signal formatted toinclude the encoded data that is stored on the processor-readable mediumof this apparatus.

The above discussion describes a method for analyzing a target syntaxelement and identifying all, if any, acceptable alternative values thatcould be substituted by replacing a block of bits in the coded bitstreamwith an alternative block of bits. By examining all syntax elements inan AVC coded sequence, we can build a list of those for which thereexists at least one acceptable alternative value. This is the list of“changeable syntax elements”. For each syntax element that can bechanged, we can build a list of the acceptable alternative values. Notethat other implementations may change syntax elements other thanmacroblock type, and the underlying intra-prediction mode. Such othersyntax elements include, for example, motion vectors, residue values orthe encoding of residue values using DCT coefficients (for example),inter-prediction indicators of a reference picture or reference frame.

Various syntax elements relate to reference information, which meansinformation that identifies a reference in some way. A reference refersgenerally to a set of information that is used as a basis for aprediction. A reference identifier generally refers to information thatidentifies a reference. For example, a reference may be identified by anintra-prediction mode of a block because the intra-prediction modeidentifies the pixels used as the basis for a prediction of a currentblock. The intra-prediction mode also identifies the manner in whichthose identified pixels are used. A reference also may be identified by,for example, a motion vector, or an indicator of a previous frame ormacroblock that is used as a basis for a prediction. Note that a givenmacroblock may have multiple reference identifiers and multiplereferences.

It should be clear that a reference, and/or a reference identifier, mayexist for intra-predicted blocks and inter-predicted blocks.Intra-predicted blocks may use a reference identifier such as, forexample, an intra-prediction mode or a macroblock type (which identifiesthe intra-prediction mode), and the reference may be, for example, ablock or a set of pixels from the same picture. Inter-predicted blocksmay use a reference identifier such as, for example, a motion vector,and the reference may be all or part of a different picture.

The goal of information embedding is to modify a subset of the syntaxelements on the list of all changeable syntax elements, changing theirvalues from their original values to one of the listed acceptablealternative values, in accordance with the payload. In one practicalimplementation, this is accomplished in three steps as shown above inFIG. 9.

As discussed above, the payload can be recovered from the AVC bitstreamwith the aid of the detection metadata. In some implementations, thedetection metadata indicates the specific syntax elements that representthe payload information and the original values of those elements. Ifthe embedding process used the original values to represent a ‘0’ andthe alternative value to represent a ‘1’, the detector can compare thevalue in the bitstream with the original value stored in the detectiondata. If they match, the detector reports a ‘0’ bit. If they do notmatch, the detector reports a ‘1’ bit. If the embedding process replacedthe original value with one of two alternative values, those twoalternative values and their corresponding labels are recovered from thedetection data. The detector then compares the value in the bitstreamwith one or more of the two alternatives and reports the correspondinglabel.

The payload data can also be recovered from the decoded pixel-domainimagery. This process is specific to the particular measure used in thesubset selection process. In one practical implementation, the measureis the mean luminance of a block of pixels. The original mean luminanceof each modified block is recovered from the detection data. Thedetector calculates the mean luminance of the specified block of pixelsin the decoded image sequence and compares that value to the originalvalue stored in the detection data. If the calculated value is higherthan the original value, the detector reports a ‘1’ bit. If thecalculated value is lower than the original, the detector reports a ‘0’bit. Such an implementation stores the location and original value inthe detection data, but does not need to store the expected detectionvalues for a ‘0’ bit or a ‘1’ bit.

This recovery method could have problems if the decoded imagery ismodified by a uniform change in luminance after decoding, but prior todetection. In order to address this, the mean luminance of a number ofreference blocks can be stored in the detection data. These are blocksthat are not changed by the embedding process. The detector cancalculate the mean luminance of the reference blocks just prior torecovery and discover any luminance changes that are unrelated to theembedding process. These luminance changes can be compensated byadjusting the original values accordingly. At least one implementationadjusts for non-uniform luminance changes by using a reference blockclose to the watermarked block.

It should be clear that information embedding may include a variety ofapplications. One such application is watermarking in which a payload isembedded in a datastream. However, various information embeddingapplications may use different requirements. For example, it may bedesirable to make a change to the datastream that results in a changethat is perceivable by a user.

Referring to FIG. 11, a general process 1100 is shown for recoveringpayload information, or more generally, for detecting a watermark. Theprocess 1100 may be applied to, for example, a coded and watermarkedbitstream, or baseband data from a decoded watermarked bitstream.

The process 1100 includes determining a location to check for a possiblewatermark (1110). Note that this process may be applied to content thatmay or may not have had a watermarking process applied. Further, even ifa watermarking process was applied, a payload bit may be embeddedwithout changing the content (as previously described) Thus, thelocation that is checked may or may not have been modified. If thecontent has been previously watermarked, then this process is designedto recover the embedded payload. The process 1100 includes accessing(1120) and analyzing (1130) data from the determined location. Theprocess 1100 further includes identifying the watermark information, ifany watermark exists, based on the analysis (1140). The identifiedwatermark information may be, for example, a sequence of bits (or otherunit of information) or a detection value determined for a macroblock(or other unit of a picture).

The process 1100 may be repeated for one or more other locations toidentify one or more additional bits that make up a payload. Thus, anentire payload may be recovered using the process 1100.

Implementations of the process 1100 include analyzing a modifiedbitstream as well as analyzing baseband data (for example, pixel domaindata or audio data). For example, a bitstream may be modified bymodifying bits associated with a target element, according to one of theimplementations previously described. A watermark may be detected insuch a bitstream by accessing the appropriate bit locations and testingfor the modification at those locations. The bit locations and modifiedvalues (or possible modified values), such as found in the embeddingmetadata, may also be included in the detection data.

Alternatively, or in addition, such a modified bitstream may be decodedand possibly subject to other processing, including re-encoding. In thecase of re-encoding, such a re-encoded bitstream will not be expected toinclude the same modified target element, at the same location, as inthe original modified bitstream. However, the re-encoded bitstream maybe decoded to produce a baseband (for example, pixel-domain or audioclip) digital representation in which an effect of the originalmodification is still typically present. Thus, detection data will beuseful that identifies the frame and block number in which the originalmodification had its impact. For example, the target element may haveoriginally been a motion vector for a particular block, and the impactof modifying the motion vector may have been an increase in the meanluminance of that particular block or set of blocks. That increase inmean luminance may be preserved despite the processing of the originalmodified bitstream. However, the motion vector, and the original changeto the motion vector will typically not be exactly the same afterre-encoding.

Referring to FIG. 12, a process 1200 is shown that describes animplementation of the process 1100 for specific use with basebandimagery (that is, pixel data) and including specific operations tailoredto baseband imagery. However, if compressed data is accessed (forexample, is retrieved from storage or received in a transmission)instead of baseband imagery, we can decode the compressed data to obtainthe baseband imagery.

The process 1200 includes accessing baseband imagery (1205), andoptionally performing temporal and/or geometric registration (1210).Registration may be necessary because the detection metadata maydescribe each change by, for example, frame number and block positionwithin the frame. Accordingly, registration facilitates the properidentification of the frame number and block. There are many approachesfor obtaining this registration, and the implementation is independentof the registration approach, so various approaches may be used.

The process 1200 includes accessing the detection metadata (1215). Eachentry in the detection metadata may define a change by, for example, itsframe position and block within the frame. For each entry, a loop (1220)is performed that may generally be defined to include operations1225-1245.

For each entry in the detection metadata, the corresponding block ofpixel data is extracted from the baseband imagery (1225). From theextracted block, the detection feature is measured, calculated, orotherwise determined (1230). In one example above, the detection featureis the mean luminance, and in another example the detection feature isthe variance.

The process 1200, and the loop (1220), includes comparing the determineddetection feature to the detection metadata (1235). This comparisonincludes, in one implementation, comparing the determined feature, forexample a mean luminance value, to two mean luminance values in thedetection metadata. The best match is selected and its correspondingsymbol is output (1240). One implementation identifies the best match asthe stored detection value that is closest to the determined detectionfeature. Other implementations may apply, for example, a non-linearevaluation to determine the best match.

The process 1200, and the loop (1220), further includes looping back togo the next intra-prediction coded block that is indicated in thedetection metadata (1245). The loop (1220) repeats for all blocksindicated in the detection metadata.

Each iteration through the loop (1220) outputs a symbol from theoperation (1240), and the result is a symbol stream (1250). The symbolstream is, optionally, decoded (1255). Note that implementations neednot have encoded the payload bits. If a data coding process was appliedto the payload (for example, as in data coder 120 in FIG. 1), then thecorresponding data decoding process is applied to the symbol streamyielding the recovered payload which is referred to as the payloadestimate (1260).

The payload estimate (1260) corresponds to the decoded symbol stream, or(if the payload was not coded) to the symbol stream. In typicalimplementations, the symbols output from the operation (1240) are bits,but other implementations may encode symbols other than bits.

As discussed above, when mean luminance is used as the detectionfeature, the system can become confused by global changes in brightness.In other words, a global increase in brightness can cause the luminanceextracted from the baseband imagery to always be closer to the brighterof the two values stored in the metadata. Thus, the symbol stream may(erroneously) consist of all ‘1’ bits. To counter this, an embedder canadd a number of reference entries into the detection information. Areference entry lists a frame number, block position, and originalluminance value of the block for a block that is not affected by thewatermarking process. The detector can read out the reference values andcompare these to the corresponding values seen in the baseband imagery.The baseband imagery can then be adjusted (perhaps during registration)such that the measured luminance in the reference blocks matches thatlisted in the detection metadata file.

Various implementations of the process 900 or the process 1200 usestored detection metadata that includes one or more of a variety ofpieces of data. For example, the detection metadata may include locationinformation identifying the locations to check for a possible watermark.The location information may include, for example, a frame identifierand/or a block identifier.

Detection metadata may include, for example, a benchmark value for afeature. The benchmark may be, for example, the value of the featurebefore the location was modified. The benchmark may be, for example,compared to the actual value (the new benchmark) of the feature for theaccessed location (1120) to provide an indication of the change. Forexample, the comparison may indicate whether the value (the benchmark)has increased or decreased as a result of the modification.

Detection metadata may include, for example, a base value for a featurefrom a different (non-watermarked) location. Such a base value may be,for example, the mean luminance of a non-watermarked location. The basevalue may be used, as explained earlier, to determine if there was achange to the data that affected more than the watermarked location. Incertain circumstances, there may have been a global change (for example,a change to an entire frame). For example, the mean luminance of anentire frame may have been changed. The base value can then be comparedto the new value of the feature at the non-watermarked location (a newbase value). If the difference is more than a threshold, then it may bedetermined that a global change occurred. Alternatively, the differencemay simply be taken to be a global difference, and the difference may beadded to the value (the new benchmark) determined for the (possible)watermarked location to account for the global change. Note that in thisalternative, the difference may (alternatively) be added to thebenchmark instead of the value (the new benchmark). Further, in certainimplementations a ratio may be determined between the base value and thenew base value, and either the benchmark or the new benchmark may bemultiplied by the ratio.

Referring to FIG. 13, a system 1300 provides a player 1305 forwatermarking video (other types of content may also be watermarked)using pre-prepared embedding information (referred to as embeddingmetadata). The watermarked video is provided for display. The system1300 also provides for re-encoding the video that is provided fordisplay, such as may occur, for example, when an unauthorized copy ofthe video is re-encoded. The system 1300 further provides for decodingthe re-encoding, and for detecting the original payload at one or morepoints in the overall system. The system 1300 also serves as a flowchart for one or more processes.

The player 1305 includes a payload 1310, a video 1315, and embeddingmetadata 1318. The player 1305 includes a watermarking unit 1320 thatreceives the payload 1310, the video 1315, and the embedding metadata1318, and that provides a watermarked video. The payload may be detectedfrom the watermarked video by a payload detector 1345.

The player 1305 includes an entropy decoder 1325 that receives thewatermarked video and performs entropy decoding. The player 1305 furtherincludes a predictive decoder 1330 that receives the entropy decodedstream from the entropy decoder 1325 and performs predictive decoding toproduce a pixel-domain image 1335. The pixel-domain image may beprovided to a display 1355, or to a payload detector 1350 that operateson baseband data.

The pixel-domain image also may be provided to a predictive re-encoder1360 followed by an entropy re-encoder to re-encode the pixel-domainimage into an encoded output 1370. The encoded output 1370 may be, forexample, an AVC-compatible bitstream. The encoded output 1370 may bestored, for example, on a recordable medium for distribution. Thepredictive re-encoder 1360 and the entropy re-encoder 1365 may be partof a device that is separate from the player 1305 and that is forproducing unauthorized copies of content (represented by the output1370).

The encoded output 1370 may be provided, for example, to an entropydecoder 1380 (which may be the same as the entropy decoder 1325)followed by a predictive decoder 1385 (which may be the same as thepredictive decoder 1330) to decode the encoded output 1370 into apixel-domain image 1390. The pixel-domain image 1390 may be provided toa display 1392 and/or to a payload detector 1395 (which may be the sameas payload detector 1350). The entropy decoder 1380 and the predictivedecoder 1385 may be part of a device separate from the player 1305 andthe predictive re-encoder 1360 and the entropy re-encoder 1365. Forexample, one such separate device may be geographically located at apayload detection facility that receives and analyzes unauthorizedcopies (represented by the output 1370) of content.

Clearly the process 900, or other processes described in thisapplication, may be performed by a variety of devices, includingprocessing devices. Such devices may also include a storage device forstoring data, such as, for example, the accessed data, the benchmarkvalue, the base value, and the location(s) of possible watermarkinformation. Further, the process 900, or other processes described inthis application, may be embodied in a set of instructions as well.

Features and aspects of described implementations may be applied tovarious applications. Applications include, for example, watermarkingdiscs such as, for example, DVDs or other discs, and watermarkingdigital streams. Watermarking of discs may be performed in variousmanners, for example, at duplication of the discs. Watermarking ofdigital streams may also be performed in various manners, for example,at playback, broadcast, download, or other transfer of a digital stream.These features and aspects may also be adapted for other applicationareas. For example, a coded bitstream could be modified for purposesother than watermarking. One example includes embedding a messageintended for a viewer to see. Further, the message could be varied basedon a payload or other criterion. Further, although the describedimplementations focused primarily on variable length coding, otherapplications may apply the same concepts to other coding techniques.

The concepts, implementations, and features described in thisapplication may be applied, for example, at various stages in a typicalDVD creation, distribution, and playback process. For example, theconcepts and features may be employed in:

1. Pre-processing the content to determine the replacement values.Replacement values need not be inserted at this stage, but may bestored, for example, in a data structure (for example, a table) forfuture insertion.

2. The data structure mentioned in item 1 above, or a signal conveyingthe data structure, or a processor-readable medium that stores the datastructure. Such a medium may contain, for example, both the datastructure and the unmodified content.

3. Inserting the replacement values from item 1 above into the content.

4. The modified content produced from item 3 above, either in a datastructure, a signal conveying the modified content, or aprocessor-readable medium storing the modified content.

5. Decoding the modified content of item 4 above to produce basebanddata, such as, for example, pixel data (if images are encoded) or audiodata (if audio is encoded).

6. The decoded content itself from item 5 above, which is modifiedcontent that may include a watermark. The decoded content may be, forexample, in a data structure, a signal conveying the decoded content, ora processor-readable medium storing at least temporarily the decodedcontent.

7. Displaying, or more generally presenting, the decoded content of item6 above. Also includes providing for display or presentation.

8. Re-encoding the decoded content from item 6 above, such that thedetection feature (that resulted from the modification) is retained eventhough the encoded bits might be different from those provided in item 4above.

9. A data structure containing the re-encoded content from item 8. Or asignal formatting and conveying the re-encoded content. Or aprocessor-readable medium storing the re-encoded content.

10. Decoding the re-encoded content of item 8 above to produce basebanddata.

11. The baseband data produced by decoding in item 10 above. Thebaseband data may be, for example, in a data structure, a signalconveying the baseband data, or a processor-readable medium storing atleast temporarily the baseband data.

12. Displaying, or more generally presenting, the baseband data of item11 above. Also includes providing for display or presentation.

13. Detecting a payload (for example, a watermark) from the modifiedcontent of item 4 above.

14. Detecting a payload from the baseband data of item 5 above.

15. Detecting a payload, if the encoded bits allow, from the re-encodedcontent of items 8 and 9 above.

16. Detecting a payload from the baseband data of item 11 above.

Note that if headers are used for various sections of this disclosure,then the header of a given section is not to be construed as limitingthe disclosure of that section to the topic of the header, nor aslimiting the disclosure of other sections to topics other than that ofthe header. Such headers are exemplary, and are intended as a generalaid to the reader. Such headers are not intended to constrain the flowof the disclosure nor to restrict the applicability or generality of thedisclosure.

A watermark, as used in this application, may refer to various items.For example, a watermark may refer to data modified or inserted to allowsubsequent detection, or to the resulting baseband modification.

As should be clear, a syntax element may represent one or more ofvarious different types of information. For example, a syntax elementmay be a motion vector, a residue DCT coefficient, a macroblock type, aquantization parameter, or a flag. Accordingly, syntax elements need notall have the same length but can have various different lengths. Variousimplementations disclosed in this application may work on any type ofsyntax element, regardless of what type of information is represented bythe syntax element and regardless of the size/length of the syntaxelement.

Various implementations are described in terms of a picture, a frame, ora block, for example. However, the concepts, implementations, andfeatures described in this application generally apply to an image, andthat image may be (or include) one or more pixels, blocks, slices,fields, frames, pictures, or sequences.

Additionally, many implementations are described in terms of video data.However, the concepts, implementations, and features are generallyequally applicable to audio data and other types of coded data.

Two devices or components are said to be coupled if the two devices aredirectly connected or indirectly connected. Directly connected deviceshave no intervening devices between the connected devices. Indirectlyconnected devices have one or more intervening devices between theconnected devices.

Further, many implementations are described with respect to AVC.However, the concepts, implementations, and features (1) need not berestricted to a standard-environment, but may be applied to environmentsin which no standard is used, (2) are certainly not restricted to AVC,(3) generally apply to other known standards, and (4) will generally beapplicable in some respect at least to future-developed standards. Otherstandards may include AVC-related standards which refer generally toother standards that are based on, or are similar to, AVC (with orwithout its extensions).

The implementations described herein may be implemented in, for example,a method or process, an apparatus, or a software program. Even if onlydiscussed in the context of a single form of implementation (forexample, discussed only as a method), the implementation of featuresdiscussed may also be implemented in other forms (for example, anapparatus or program). An apparatus may be implemented in, for example,appropriate hardware, software, and firmware. The methods may beimplemented in, for example, an apparatus such as, for example, aprocessor, which refers to processing devices in general, including, forexample, a computer, a microprocessor, an integrated circuit, or aprogrammable logic device. Processing devices also include communicationdevices, such as, for example, computers, cell phones, portable/personaldigital assistants (“PDAs”), and other devices that facilitatecommunication of information between end-users.

Implementations of the various processes and features described hereinmay be embodied in a variety of different equipment or applications,particularly, for example, equipment or applications associated withdata encoding and decoding, or equipment or applications associated withcontent production. Examples of equipment include video coders, videodecoders, video codecs, web servers, set-top boxes, laptops, personalcomputers, cell phones, PDAs, and other communication devices. As shouldbe clear, the equipment may be mobile and even installed in a mobilevehicle.

Additionally, the methods may be implemented by instructions beingperformed by a processor, and such instructions may be stored on aprocessor-readable medium such as, for example, an integrated circuit, asoftware carrier or other storage device such as, for example, a harddisk, a compact diskette, a random access memory (“RAM”), or a read-onlymemory (“ROM”). The instructions may form an application programtangibly embodied on a processor-readable medium. Instructions may be,for example, in hardware, firmware, software, or a combination.Instructions may be found in, for example, an operating system, aseparate application, or a combination of the two. A processor may becharacterized, therefore, as, for example, both a device configured tocarry out a process and a device that includes a processor-readablemedium having instructions for carrying out a process.

As should be evident to one of skill in the art, implementations mayalso produce a signal formatted to carry information that may be, forexample, stored or transmitted. The information may include, forexample, instructions for performing a method, or data produced by oneof the described implementations. Such a signal may be formatted, forexample, as an electromagnetic wave (for example, using a radiofrequency portion of spectrum) or as a baseband signal. The formattingmay include, for example, encoding a data stream and modulating acarrier with the encoded data stream. The information that the signalcarries may be, for example, analog or digital information. The signalmay be transmitted over a variety of different wired or wireless links,as is known.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. For example,elements of different implementations may be combined, supplemented,modified, or removed to produce other implementations. Additionally, oneof ordinary skill will understand that other structures and processesmay be substituted for those disclosed and the resulting implementationswill perform at least substantially the same function(s), in at leastsubstantially the same way(s), to achieve at least substantially thesame result(s) as the implementations disclosed. Accordingly, these andother implementations are contemplated by this application and arewithin the scope of the following claims.

The invention claimed is:
 1. A method for determining a watermark, themethod comprising: accessing variable-length encoded data including anencoding of information identifying an actual reference used topredictively encode an image, the encoding of the information having aparticular value, and the variable-length encoded data also including anencoding of a residue produced from predictively encoding the imagebased on the actual reference; and determining an alternative value thatis different from the particular value, such that the alternative valueis an encoding of alternative information that identifies a differentreference from the actual reference, wherein the alternative value isfor use in watermarking the variable-length encoded data by insertingthe alternative value into the variable-length encoded data as asubstitute for the particular value and by also retaining in thevariable-length encoded data the residue produced from predictivelyencoding the image based on the actual reference.
 2. The method of claim1 further comprising designating the alternative value as a substitutefor the particular value for use in watermarking the variable-lengthencoded data.
 3. The method of claim 2 further comprising evaluatingeffectiveness of using the alternative value, in place of the currentvalue, as a watermark, and wherein designating the alternative value asa substitute is based on a result of evaluating effectiveness.
 4. Themethod of claim 3 wherein evaluating comprises determining a cost forthe alternative value based on fidelity of the watermark and robustnessof the watermark.
 5. The method of claim 3 further comprising:evaluating effectiveness of an additional alternative value; anddetermining, based on results from evaluating the alternative value andevaluating the additional alternative value, that the alternative valueis more effective, and wherein designating the alternative value as asubstitute is based on determining that the alternative value is moreeffective.
 6. The method of claim 5 wherein: the particular valueidentifies a first value for a macroblock type field for anintra-predicted 16×16 macroblock in H.264/AVC or a related standard, thefirst value indicating a first intra-prediction mode, the alternativevalue identifies a second value for a macroblock type field for anintra-predicted 16×16 macroblock in H.264/AVC or a related standard, thesecond value indicating a second intra-prediction mode, and theadditional alternative value identifies a third value for a macroblocktype field for an intra-predicted 16×16 macroblock in H.264/AVC or arelated standard, the third value indicating a third intra-predictionmode.
 7. The method of claim 6 wherein a CAVLC encoding of each of thefirst value, the second value, and the third value uses the same numberof bits.
 8. The method of claim 1 further comprising: inserting into atable the alternative value; determining location of the particularvalue in the variable-length encoded data; and inserting into the tableinformation identifying the location in the variable-length encoded dataat which the particular value is located, the location being where thealternative value would be inserted into the variable-length encodeddata to replace the particular value.
 9. The method of claim 1 furthercomprising replacing the particular value with the alternative value inthe variable-length encoded data.
 10. The method of claim 1 wherein theinformation identifying the actual reference identifies pixel locationsof blocks that are adjacent to a current block, wherein the values ofthe pixel locations are used to predict at least a portion of thecurrent block.
 11. The method of claim 1 wherein the informationidentifying the actual reference identifies a reference block or areference picture.
 12. The method of claim 1 wherein the informationidentifying the actual reference comprises a macroblock type field inH.264/AVC or a related standard.
 13. The method of claim 1 wherein theinformation identifying the actual reference comprises a macroblock typefield for an intra-predicted 16×16 macroblock in H.264/AVC or a relatedstandard.
 14. The method of claim 1 wherein the information identifyingthe actual reference comprises a prediction mode in H.264/AVC or arelated standard.
 15. The method of claim 1 wherein the informationidentifying the actual reference comprises a motion vector.
 16. Themethod of claim 1 wherein the variable-length encoded data furtherincludes an encoding of information identifying a current residueresulting from predicting the image with the current reference.
 17. Themethod of claim 1 wherein: the variable-length encoded data has beenencoded using a particular variable-length code, and the alternativevalue encodes the alternative information using the particularvariable-length code.
 18. An apparatus for determining a watermark, theapparatus comprising: an entropy decoder to decode variable-lengthencoded data including an encoding of information identifying an actualreference used to predictively encode an image, the encoding of theinformation having a particular value, and the variable-length encodeddata also including an encoding of a residue produced from predictivelyencoding the image based on the actual reference; and a watermarkgenerator, coupled to the entropy decoder, to determine an alternativevalue that is different from the particular value, such that thealternative value is an encoding of alternative information thatidentifies a different reference from the actual reference, wherein thealternative value is for use in watermarking the variable-length encodeddata by inserting the alternative value into the variable-length encodeddata as a substitute for the particular value and by also retaining inthe variable-length encoded data the residue produced from predictivelyencoding the image based on the actual reference.
 19. The apparatus ofclaim 18 wherein the watermark generator is configured to createmetadata for use in watermarking the variable-length encoded data, themetadata including (1) the alternative value and (2) a location of theparticular value in the variable-length encoded data, and the metadataassociating the alternative value and the location.
 20. An apparatus fordetermining a watermark, the apparatus comprising: means for accessingvariable-length encoded data including an encoding of informationidentifying an actual reference used to predictively encode an image,the encoding of the information having a particular value, and thevariable-length encoded data also including an encoding of a residueproduced from predictively encoding the image based on the actualreference; and means for determining an alternative value that isdifferent from the particular value, such that the alternative value isan encoding of alternative information that identifies a differentreference from the actual reference, wherein the alternative value isfor use in watermarking the variable-length encoded data by insertingthe alternative value into the variable-length encoded data as asubstitute for the particular value and by also retaining in thevariable-length encoded data the residue produced from predictivelyencoding the image based on the actual reference.
 21. A processorconfigured to perform at least the following: accessing variable-lengthencoded data including an encoding of information identifying an actualreference used to predictively encode an image, the encoding of theinformation having a particular value, and the variable-length encodeddata also including an encoding of a residue produced from predictivelyencoding the image based on the actual reference; and determining analternative value that is different from the particular value, such thatthe alternative value is an encoding of alternative information thatidentifies a different reference from the actual reference, wherein thealternative value is for use in watermarking the variable-length encodeddata by inserting the alternative value into the variable-length encodeddata as a substitute for the particular value and by also retaining inthe variable-length encoded data the residue produced from predictivelyencoding the image based on the actual reference.
 22. A non-transitoryprocessor-readable medium having stored thereon instructions forperforming at least the following: accessing variable-length encodeddata including an encoding of information identifying an actualreference used to predictively encode an image, the encoding of theinformation having a particular value, and the variable-length encodeddata also including an encoding of a residue produced from predictivelyencoding the image based on the actual reference; and determining analternative value that is different from the particular value, such thatthe alternative value is an encoding of alternative information thatidentifies a different reference from the actual reference, wherein thealternative value is for use in watermarking the variable-length encodeddata by inserting the alternative value into the variable-length encodeddata as a substitute for the particular value and by also retaining inthe variable-length encoded data the residue produced from predictivelyencoding the image based on the actual reference.
 23. A non-transitoryprocessor-readable medium having watermark data stored thereon, thewatermark data for use in watermarking variable-length encoded data thatincludes an encoding of information identifying an actual reference usedto predictively encode an image, the encoding of the information havinga particular value, and the variable-length encoded data also includingan encoding of a residue produced from predictively encoding the imagebased on the actual reference, and the watermark data including analternative value that is different from the particular value, such thatthe alternative value is an encoding of alternative information thatidentifies a different reference from the actual reference, thealternative value being a substitute for the particular value for use inwatermarking the variable-length encoded data by inserting thealternative value into the variable-length encoded data as a substitutefor the particular value and by also retaining in the variable-lengthencoded data the residue produced from predictively encoding the imagebased on the actual reference.
 24. The processor-readable medium ofclaim 23 wherein the processor-readable medium has stored thereon thevariable-length encoded data that includes the encoding of informationidentifying the actual reference used to predictively encode the image,the encoding of the information having the particular value.
 25. Amethod for watermarking, the method comprising: accessingvariable-length encoded data including an encoding of informationidentifying an actual reference used to predictively encode an image,the encoding of the information having a particular value, and thevariable-length encoded data also including an encoding of a residueproduced from predictively encoding the image based on the actualreference; accessing an alternative value that is different from theparticular value, such that the alternative value is an encoding ofalternative information that identifies a different reference from theactual reference; and inserting the alternative value into thevariable-length encoded data as a substitute for the particular value towatermark the image, wherein the variable-length encoded data includesthe alternative value encoding the alternative information identifyingthe different reference and also includes the residue produced frompredictively encoding the image based on the actual reference.
 26. Themethod of claim 25 wherein: accessing the variable-length encoded dataoccurs in response to a request for presentation of the variable-lengthencoded data, and processing operations to determine the alternativevalue have been performed prior to the accessing of the variable-lengthencoded data, such that upon receiving the request for presentation ofthe variable-length encoded data and subsequently accessing thevariable-length encoded data, the alternative value may be insertedwithout needing to determine the alternative value.
 27. The method ofclaim 25 further comprising performing processing operations todetermine the alternative value, prior to accessing the alternativevalue.
 28. An apparatus comprising a stream modification unit configuredto perform at least the following: accessing variable-length encodeddata including an encoding of information identifying an actualreference used to predictively encode an image, the encoding of theinformation having a particular value, and the variable-length encodeddata also including an encoding of a residue produced from predictivelyencoding the image based on the actual reference; accessing analternative value that is different from the particular value, such thatthe alternative value is an encoding of alternative information thatidentifies a different reference from the actual reference; andinserting the alternative value into the variable-length encoded data asa substitute for the particular value to watermark the image, whereinthe variable-length encoded data includes the alternative value encodingthe alternative information identifying the different reference alsoincludes the residue produced from predictively encoding the image basedon the actual reference.
 29. The apparatus of claim 28 furthercomprising a DVD reader, and wherein the apparatus is a DVD player. 30.An apparatus for watermarking, the apparatus comprising: means foraccessing variable-length encoded data including an encoding ofinformation identifying an actual reference used to predictively encodean image, the encoding of the information having a particular value andthe variable-length encoded data also including an encoding of a residueproduced from predictively encoding the image based on the actualreference; means for accessing an alternative value that is differentfrom the particular value, such that the alternative value is anencoding of alternative information that identifies a differentreference from the actual reference; and means for inserting thealternative value into the variable-length encoded data as a substitutefor the particular value to watermark the image wherein thevariable-length encoded data includes the alternative value encoding thealternative information identifying the different reference and alsoincludes the residue produced from predictively encoding the image basedon the actual reference.
 31. A non-transitory processor configured toperform at least the following: accessing variable-length encoded dataincluding an encoding of information identifying an actual referenceused to predictively encode an image, the encoding of the informationhaving a particular value, and the variable-length encoded data alsoincluding an encoding of a residue produced from predictively encodingthe image based on the actual reference; accessing an alternative valuethat is different from the particular value, such that the alternativevalue is an encoding of alternative information that identifies adifferent reference from the actual reference; and inserting thealternative value into the variable-length encoded data as a substitutefor the particular value to watermark the image, wherein thevariable-length encoded data includes the alternative value encoding thealternative information identifying the different reference and alsoincludes the residue produced from predictively encoding the image basedon the actual reference.
 32. A non-transitory processor-readable mediumhaving stored thereon instructions for performing at least thefollowing: accessing variable-length encoded data including an encodingof information identifying an actual reference used to predictivelyencode an image, the encoding of the information having a particularvalue, and the variable-length encoded data also including an encodingof a residue produced from predictively encoding the image based on theactual reference; accessing an alternative value that is different fromthe particular value, such that the alternative value is an encoding ofalternative information that identifies a different reference from theactual reference; and inserting the alternative value into thevariable-length encoded data as a substitute for the particular value towatermark the image, wherein the variable-length encoded data includesthe alternative value encoding the alternative information identifyingthe different reference and also includes the residue produced frompredictively encoding the image based on the actual reference.
 33. Anon-transitory processor-readable medium having stored thereonvariable-length encoded data for an encoding of an image, thevariable-length encoded data comprising an encoding of informationidentifying an alternative reference that is different from the actualreference used to predictively encode the image, wherein: theinformation identifying the alternative reference provides a watermarkfor the image, the variable-length encoded data stored on the mediumfurther comprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference, and thevariable-length encoded data includes the alternative value encoding thealternative information identifying the different reference and alsoincludes the residue produced from predictively encoding the image basedon the actual reference.
 34. A method for decoding, the methodcomprising: decoding variable-length encoded data for an encoding of animage, the variable-length encoded data comprising an encoding ofinformation identifying an alternative reference that is different froman actual reference used in predictively encoding the image, wherein theinformation identifying the alternative reference provides a watermarkfor the image, wherein the variable-length encoded data furthercomprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference.
 35. Themethod of claim 34 wherein the variable-length encoded data furthercomprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference.
 36. Themethod of claim 34 wherein decoding comprises decoding thevariable-length code and the predictive code.
 37. The method of claim 34wherein decoding produces a baseband reconstruction of the image thatvaries from the original image by including a detectable feature arisingfrom the use of the information identifying the alternative reference asa watermark.
 38. An apparatus comprising a decoder configured to decodevariable-length encoded data for an encoding of an image, thevariable-length encoded data comprising an encoding of informationidentifying an alternative reference that is different from an actualreference used in predictively encoding the image, wherein theinformation identifying the alternative reference provides a watermarkfor the image, wherein the variable-length encoded data furthercomprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference.
 39. Anapparatus comprising: means for decoding variable-length encoded datafor an encoding of an image, the variable-length encoded data comprisingan encoding of information identifying an alternative reference that isdifferent from an actual reference used in predictively encoding theimage, wherein the information identifying the alternative referenceprovides a watermark for the image, wherein the variable-length encodeddata further comprises an encoding of information identifying a residuefrom predictively encoding the image using the actual reference.
 40. Theapparatus of claim 39 further comprising memory means for storing thevariable-length encoded data.
 41. A processor configured to decodevariable-length encoded data for an encoding of an image, thevariable-length encoded data comprising an encoding of informationidentifying an alternative reference that is different from an actualreference used in predictively encoding the image, wherein theinformation identifying the alternative reference provides a watermarkfor the image, wherein the variable-length encoded data furthercomprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference.
 42. Aprocessor-readable medium having stored thereon instructions fordecoding variable-length encoded data for an encoding of an image, thevariable-length encoded data comprising an encoding of informationidentifying an alternative reference that is different from an actualreference used in predictively encoding the image, wherein theinformation identifying the alternative reference provides a watermarkfor the image, wherein the variable-length encoded data furthercomprises an encoding of information identifying a residue frompredictively encoding the image using the actual reference.
 43. Anon-transitory processor-readable medium having stored thereon apixel-domain image that includes a detectable feature arising from awatermark of a variable-length encoding of one or more syntax elementsfrom a predictive encoding of the image, the variable-length encodinghaving included an encoding of information identifying an alternativereference that is different from an actual reference used inpredictively encoding the image, wherein the information identifying thealternative reference provided the watermark for the image, wherein thevariable-length encoded data further comprises an encoding ofinformation identifying a residue from predictively encoding the imageusing the actual reference.
 44. A method comprising providing fordisplay a pixel-domain image that includes a detectable feature arisingfrom a watermark of a variable-length encoding of one or more syntaxelements from a predictive encoding of the image, the variable-lengthencoding having included an encoding of information identifying analternative reference that is different from an actual reference used inpredictively encoding the image, wherein the information identifying thealternative reference provided the watermark for the image, wherein thevariable-length encoded data further comprises an encoding ofinformation identifying a residue from predictively encoding the imageusing the actual reference.
 45. A method comprising: variable-lengthencoding a predictive encoding of a watermarked version of an image, thevariable-length encoded data comprising an indicator of a currentreference used in encoding the watermarked version of the image and anindicator of a resulting residue, the watermarked version of the imagehaving a detectable characteristic that provides a watermark, whereinthe detectable characteristic results from (1) modifying a previousvariable-length encoding of a previous predictive-encoding of the image,the previous predictive-encoding being based on a previous reference andincluding information identifying the previous reference, wherein themodifying includes distorting the information in order to indicate analternative reference and to provide the detectable characteristic forthe image, and (2) decoding the previous variable-length encoding andthe previous predictive-encoding of the image to produce the watermarkedversion of the image in which the detectable characteristic is present.46. A non-transitory processor-readable medium having stored thereon:variable-length encoded data from a variable-length encoding of apredictive encoding of a watermarked version of an image, thevariable-length encoded data comprising an indicator of a currentreference used in encoding the watermarked version of the image and anindicator of a resulting residue, the watermarked version of the imagehaving a detectable characteristic that provides a watermark, whereinthe detectable characteristic results from (1) modifying a previousvariable-length encoding of a previous predictive-encoding of the image,the previous predictive-encoding being based on a previous reference andincluding information identifying the previous reference, wherein themodifying includes distorting the information in order to indicate analternative reference and to provide the detectable characteristic forthe image, and (2) decoding the previous variable-length encoding andthe previous predictive-encoding of the image to produce the watermarkedversion of the image in which the detectable characteristic is present.47. A method comprising decoding variable-length encoded data from avariable-length encoding of a predictive encoding of a watermarkedversion of an image, the variable-length encoded data comprising anindicator of a current reference used in encoding the watermarkedversion of the image and an indicator of a resulting residue, thewatermarked version of the image having a detectable characteristic thatprovides a watermark, wherein the detectable characteristic results from(1) modifying a previous variable-length encoding of a previouspredictive-encoding of the image, the previous predictive-encoding beingbased on a previous reference and including information identifying theprevious reference, wherein the modifying includes distorting theinformation in order to indicate an alternative reference and to providethe detectable characteristic for the image, and (2) decoding theprevious variable-length encoding and the previous predictive-encodingof the image to produce the watermarked version of the image in whichthe detectable characteristic is present.
 48. A non-transitoryprocessor-readable medium having stored thereon pixel-domain image dataresulting from decoding variable-length encoded data from avariable-length encoding of a predictive encoding of a watermarkedversion of an image, the variable-length encoded data comprising anindicator of a current reference used in encoding the watermarkedversion of the image and an indicator of a resulting residue, thewatermarked version of the image having a detectable characteristic thatprovides a watermark, wherein the detectable characteristic results from(1) modifying a previous variable-length encoding of a previouspredictive-encoding of the image, the previous predictive-encoding beingbased on a previous reference and including information identifying theprevious reference, wherein the modifying includes distorting theinformation in order to indicate an alternative reference and to providethe detectable characteristic for the image, and (2) decoding theprevious variable-length encoding and the previous predictive-encodingof the image to produce the watermarked version of the image in whichthe detectable characteristic is present.
 49. A method comprisingproviding for display pixel-domain image data resulting from decodingvariable-length encoded data from a variable-length encoding of apredictive encoding of a watermarked version of an image, thevariable-length encoded data comprising an indicator of a currentreference used in encoding the watermarked version of the image and anindicator of a resulting residue, the watermarked version of the imagehaving a detectable characteristic that provides a watermark, whereinthe detectable characteristic results from (1) modifying a previousvariable-length encoding of a previous predictive-encoding of the image,the previous predictive-encoding being based on a previous reference andincluding information identifying the previous reference, wherein themodifying includes distorting the information in order to indicate analternative reference and to provide the detectable characteristic forthe image, and (2) decoding the previous variable-length encoding andthe previous predictive-encoding of the image to produce the watermarkedversion of the image in which the detectable characteristic is present.50. A method for determining payload information, the method comprising:accessing information that identifies a location in variable-lengthencoded data at which a variable-length encoding of information islocated, the information identifying one of at least two referencesassociated with a predictive encoding of an image; accessing theencoding of the information at the location in the variable-lengthencoded data, wherein the information also identifies a residueresulting from predictively encoding the image using a reference otherthan the identified reference; determining, based on the encoding of theinformation, which of the at least two references is identified; anddetermining payload information based on a result of determining whichof the at least two references is identified.
 51. The method of claim 50wherein the identified reference is different from an actual referenceused in the predictive encoding of the image, and variable-lengthencoding the identified reference rather than the actual referenceprovided a watermark for the image.
 52. A method for determining payloadinformation, the method comprising: accessing a pixel-domain image thatincludes a detectable feature arising from a watermark of avariable-length encoding of one or more syntax elements from apredictive encoding of the image, the variable-length encoding includingan encoding of information identifying an alternative reference that isdifferent from an actual reference used in predictively encoding theimage, wherein the information identifying the alternative referenceprovided the watermark for the image; accessing detection metadataindicating a location in the pixel-domain image at which the detectablefeature is located; analyzing the pixel-domain image at the location todetermine a value for the detectable feature; accessing additionaldetection metadata indicating a comparison value for the detectablefeature; comparing the determined value for the detectable feature withthe comparison value; and determining payload information based on aresult of the comparing.
 53. The method of claim 25 wherein thepredictive encoding of the image is intra-prediction, such that theactual reference is an intra-prediction reference.
 54. The method ofclaim 25 wherein the different reference is an intra-predictionreference.
 55. The method of claim 25 wherein the informationidentifying the actual reference includes a macroblock type field asused in H.264 or a related standard.
 56. The method of claim 25 whereinthe information identifying the actual reference includes anintra-prediction mode indicator, as used in H.264 or a related standard.